1. Field of the Invention
The present invention relates to a semiconductor optical functional element appropriate for use in super-high capacity optical telecommunications and super-high speed optical signal processing, and more particularly to a waveguide-type of semiconductor optical functional element, such as an electroabsorption optical modulator, a supersaturation absorption optical switch, or a wavelength switch.
2. Description of Related Art
For example, the semiconductor optical functional element disclosed in citation I: [Structural dependence of a Modulation characteristics of MQW EA Modulators, Sano et al., Autumn 1990 proceedings of the Denshi Jouhou Tsuushin Gakkai, C-156, p. 4-198] and the electroabsorption optical intensity modulator in citation II: [High-speed InGaAs/InAlAs multiple quantum well optical modulators with bandwidth in excess of 20 GHz at 1.55 .mu.m, Isamu Kotaka et al., IEEE Photon. Techno. Lett. Vol. 1, No. 5, pp. 100-101, 1989] were known.
In the structure of the electroabsorption optical intensity modulator disclosed in these references, an n-type cladding layer, i-type optical waveguide layer, p-type cladding layer, and p-type ohmic contact layer are formed in that order on an n-type semiconductor substrate; a PIN double heterojunction structure is thereby formed with the n-type layers, optical waveguide layer, and p-type layers.
When reverse voltage is applied to the PIN structure (double heterojunction structure) of an optical modulator of this type, an electrical field is applied to the optical waveguide layer, which is an intrinsic layer (i-type layer), and the absorption of the electric field results in a high coefficient of optical absorption within the optical waveguide layer. The intensity of light introduced into this optical modulator from outside the structure is modulated by the applied voltage.
Other semiconductor optical functional elements include, for example, the conventional optical phase modulator having the PIN junction structure noted in "High-speed electro-optic phase modulators using InGaAs/InAlAs multiple quantum well waveguides," Koichi Wakita et al., IEEE Photon. Techno. Lett. Vol. 1, No. 12, pp. 441-442, 1989. In such a conventional optical phase modulator, light introduced from outside can undergo phase modulation in the optical waveguide layer when the PIN junction is put in reverse bias.
In some cases, these semiconductor optical functional elements were driven by applying forward bias voltage to the PIN structure thereof. For example, in some electroabsorption optical intensity modulators, the PIN structure is put in forward bias in order to adjust the form of an eye pattern (random blocks of an applied electrical signal) or to improve the extinction ratio (ratio of minimum value to maximum value of output light intensity).
However, the following problems occur when the PIN structure is put in forward bias.
First, the intensity of the electrical field applied to the PIN structure becomes small. As a result, the capacitance (electrostatic capacitance) becomes high because the depletion layer in the PIN structure becomes thin. This causes a mismatch of impedance to occur between the optical functional element and external circuitry, such as driving circuits, logic circuits, or the like, to thereby deteriorate the high frequency response characteristics.
Also, drift speed drops because of the reduced intensity of the electrical field applied to the PIN structure. Thus, the photocarrier sweep efficiency decreases, and the number of carriers generated from the optical waveguide layer becomes greater than the number of carriers swept from the optical waveguide layer. As a result, carriers accumulate within the optical waveguide layer and change the index of refraction of the optical waveguide layer. Change of the index of refraction increases the .alpha. parameter showing frequency variation.
The deterioration of the high frequency response characteristics and the increase of the .alpha. parameter are related to the deterioration of the quality of light propagation (or transmission), for example, the distortion of the waveform of light propagated within an optic fiber.
Second, when the forward bias exceeds the built in voltage of the PIN structure, current surges within the device.
ASE (amplified spontaneous emission: current flowing into the optical waveguide layer is converted to light and emitted) is thereby caused and this light is treated together with the modulated light within the optic fiber.
Third, the current flowing into the optical waveguide layer is converted to heat and Joule heat is emitted. As a result, the temperature of the optical functional element increases, causes changes in the properties thereof, and reduces the lifespan thereof.
Sometimes a forward surge of high power is applied unexpectedly to an optical functional element. The excessive Joule heat generated at such times in the element can bring about lethal deterioration, which is related to the destruction of the element.
However, there have been no countermeasures to resolve the various problems which might arise as a result of forward biasing of an electroabsorption optical intensity modulator, optical phase modulator, or other semiconductor optical functional element having a conventional PIN structure.